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Abstract:

A protective coating encapsulates bond pads disposed on a substrate of an
optical communications module and extends in between the bond pads. The
protective coating has characteristics that (1) increase the dielectric
resistances between adjacent bond pads on the substrate, (2) protect the
bond pads from moisture in the environment, and (3) prevents, or at least
reduces, ion migration between adjacent bond pads. In this way, the
protective coating prevents, or at least reduces, corrosion growth that
can lead to impedance degradation and electrical shorts between adjacent
bond pads.

Claims:

1. An optical communications module comprising: a substrate having at
least first and second electrically-conductive bond pads disposed on a
surface of the substrate, the first and second bond pads being
electrically coupled to respective electrical conductors disposed in or
on the substrate; at least one integrated circuit (IC) having at least
first and second electrically-conductive bond pads disposed thereon; at
least first and second electrically-conductive bond wires, each bond wire
having a first end and a second end, the first end of the first bond wire
being attached to the first bond pad disposed on said at least one IC,
the second end of the first bond wire being attached to the first bond
pad disposed on the surface of the substrate, the first end of the second
bond wire being attached to the second bond pad disposed on said at least
one IC, the second end of the second bond wire being attached to the
second bond pad disposed on the surface of the substrate; and a
protective coating encapsulating at least the first and second bond pads
disposed on the surface of the substrate and extending in between the
first and second bond pads disposed on the surface of the substrate, the
protective coating electrically isolating the first and second bond pads
disposed on the surface of the substrate from each other.

2. The optical communications module of claim 1, wherein the protective
coating also prevents moisture in the environment from coming into
contact with the first and second bond pads disposed on the surface of
the substrate.

3. The optical communications module of claim 2, wherein the protective
coating also prevents ions from migrating between the first and second
bond pads disposed on the surface of the substrate.

5. The optical communications module of claim 1, wherein a spacing, or
pitch, between the first and second bond pads disposed on the surface of
the substrate is between about 15 and 40 micrometers (microns).

6. The optical communications module of claim 5, wherein the spacing, or
pitch, between the first and second bond pads disposed on the surface of
the substrate is about 20 microns.

10. A method for encapsulating bond pads disposed on a substrate of an
optical communications module in a protective coating, the method
comprising: providing a substrate having at least first and second
electrically-conductive bond pads disposed on a surface of the substrate,
the first and second bond pads being electrically coupled to respective
electrical conductors disposed in or on the substrate; performing a die
attachment process to attach at least one integrated circuit (IC) to a
surface of the substrate or to a surface of a mounting structure mounted
on the substrate, said at least one IC having at least first and second
electrically-conductive bond pads disposed thereon; performing a wire
bonding process to electrically interconnect the first and second bond
pads on the surface of the substrate with the first and second bond pads
disposed on said at least one IC by first and second
electrically-conductive bond wires, respectively, each bond wire having a
first end and a second end, the first end of the first bond wire being
attached to the first bond pad disposed on said at least one IC, the
second end of the first bond wire being attached to the first bond pad
disposed on the surface of the substrate, the first end of the second
bond wire being attached to the second bond pad disposed on said at least
one IC, the second end of the second bond wire being attached to the
second bond pad disposed on the surface of the substrate; and performing
an encapsulation process to encapsulate at least the first and second
bond pads disposed on the surface of the substrate with a protective
coating, the protective coating extending in between the first and second
bond pads disposed on the surface of the substrate, and wherein the
protective coating electrically isolates the first and second bond pads
disposed on the surface of the substrate from each other.

11. The method of claim 10, wherein the protective coating also prevents
moisture in the environment from coming into contact with the first and
second bond pads disposed on the surface of the substrate.

12. The method of claim 11, wherein the protective coating also prevents
ions from migrating between the first and second bond pads disposed on
the surface of the substrate.

13. The method of claim 10, wherein the protective coating comprises an
epoxy.

14. The method of claim 10, wherein a spacing, or pitch, between the
first and second bond pads disposed on the surface of the substrate is
between about 15 and 40 micrometers (microns).

15. The method of claim 14, wherein the spacing, or pitch, between the
first and second bond pads disposed on the surface of the substrate is
about 20 microns.

[0002] In optical communications networks, optical transmitter modules,
optical receiver modules, and optical transceiver modules are used to
transmit and receive optical signals over optical fibers. In a transmit
portion of such an optical module, a laser generates modulated optical
signals that represent data, which are then transmitted over an optical
fiber. The laser can be, for example, a Vertical Cavity Surface Emitting
Laser (VCSEL) or an edge-emitting laser. In a receive portion of such a
module, an optics system directs light propagating out of the end of an
optical fiber onto an optical detector or photodetector, which converts
the optical energy into electrical energy. A photodetector is typically a
semiconductor photodiode device, such as a PIN (p-type/intrinsic/n-type)
photodiode. Optical transceiver modules typically include multiple lasers
for transmitting multiple data signals and multiple photodiodes for
receiving multiple data signals.

[0003] An optical module is commonly assembled by mounting the
optoelectronic device, i.e., laser or optical detector, on a substrate,
also referred to as a leadframe. As the optoelectronic device typically
comprises a microelectronic semiconductor die, electrical connections
between the die and conductors on the substrate are made by a technique
known as wirebonding. Wirebonding is a technique in which one end of a
very fine wire, known as a bond wire, is bonded to a bond pad on the die
using thermal or ultrasonic energy, and the other end is bonded to a bond
pad on the substrate. A lens assembly can be aligned with the transmit or
receive optical ports of the die and mounted in fixed relation to the die
and substrate.

[0004] Bond wires are extremely fragile because the wires are extremely
fine, i.e., very thin gauge. Rough handling of the optical assembly can
easily break or dislodge a bond wire. In some optical assemblies, the
bond wires are protected by an enclosure or module body that encloses the
entire optical assembly. In some assemblies, the bond wires are
encapsulated in a dielectric resin to protect them from external forces
that can break or dislodge them.

[0005] While the aforementioned techniques may be effective at protecting
the bond wires from external forces that can break or dislodge them, they
are not aimed at, and are not effective at, protecting the bond wires
and/or the bond pads from electro-chemical reactions/ion migration that
can degrade the integrity of the bond wires and/or of the bond pads. A
need exists for such a solution.

SUMMARY OF THE INVENTION

[0006] The invention provides an optical communications module having bond
pads that are encapsulated in a protective coating and a method of
encapsulating the bond pads in the protective coating. The optical
communications module comprises a substrate having at least first and
second bond pads on a surface thereof, at least one integrated circuit
(IC), at least first and second electrically-conductive bond wires, and a
protective coating. Each bond wire has a first end and a second end. The
first end of the first bond wire is attached to the first bond pad
disposed on the IC and the second end of the first bond wire is attached
to the first bond pad disposed on the surface of the substrate. The first
end of the second bond wire is attached to the second bond pad disposed
on the IC and the second end of the second bond wire is attached to the
second bond pad disposed on the surface of the substrate. The protective
coating encapsulates at least the first and second bond pads disposed on
the surface of the substrate and extends in between the first and second
bond pads disposed on the surface of the substrate. The protective
coating electrically isolates the first and second bond pads disposed on
the surface of the substrate from each other.

[0007] The method comprises providing a substrate, performing a die
attachment process to attach at least one IC to a surface of the
substrate or to a surface of a mounting structure mounted on the
substrate, performing a wire bonding process to electrically interconnect
the first and second bond pads on the surface of the substrate with the
first and second bond pads disposed on the IC by first and second
electrically-conductive bond wires, respectively, and performing an
encapsulation process to encapsulate at least the first and second bond
pads disposed on the surface of the substrate with a protective coating
that extends in between the first and second bond pads disposed on the
surface of the substrate. The protective coating electrically isolates
the first and second bond pads disposed on the surface of the substrate
from each other.

[0008] These and other features and advantages of the invention will
become apparent from the following description, drawings and claims.

BRIEF DESCRIPTION OF THE DRAWING

[0009] FIG. 1 illustrates a top perspective view of a parallel optical
communications module in accordance with an illustrative embodiment
having a protective coating thereon that encompasses bond pads disposed
on a substrate of the module.

[0010]FIG. 2 illustrates a top plan view of the parallel optical
communications module shown in FIG. 1 having the protective coating
thereon that encompasses the bond pads disposed on the substrate of the
module.

[0011]FIG. 3 illustrates a front plan view of the parallel optical
communications module shown in FIG. 1 having the protective coating
thereon that encompasses the bond pads disposed on the substrate of the
module.

[0012]FIG. 4 illustrates a side plan view of the parallel optical
communications module shown in FIG. 1 having the protective coating
thereon that encompasses the bond pads disposed on the substrate of the
module.

[0013] FIG. 5 illustrates a cross-sectional view of a portion of the
parallel optical transmitter module shown in FIGS. 1-4 that demonstrates
the manner in which the protective coating encapsulates the bond pads
disposed on the upper surface of the substrate and the ends of the bond
wires that are attached to the bond pads.

[0014] FIG. 6 illustrates a flowchart that demonstrates the method for
encapsulating the bond pads in a protective coating.

DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT

[0015] In parallel optical communications modules, there are relatively
large numbers of optoelectronic devices (e.g., laser diodes or
photodiodes) for transmitting or receiving optical signals over
respective optical channels. Each optoelectronic device is electrically
connected to at least one integrated circuit (IC) die. If the
optoelectronic device is a photodiode, the IC die is typically a receiver
IC die for processing the electrical signal produced by the photodiode.
If the optoelectronic device is a laser diode, the IC die is typically a
driver IC die for producing an electrical drive signal that drives the
laser diode.

[0016] In parallel optical communications modules that have a large number
of channels, the bond wires that electrically connect bond pads on the IC
die to respective bond pads on the substrate are very close together due
to the fact that the pitch, or distance, between adjacent bond pads is
very small. In accordance with the invention, it has been discovered that
this very small pitch can result in corrosion growth on some adjacent
bond wires and bond pads on the substrate near where the ends of the bond
wires are attached to the bond pads on the substrate. For example, in
some cases, some of the adjacent bond wires are inter-IC lines, known as
I2C lines, which are used for serial, synchronous communications
between the substrate and the driver or receiver IC. There are typically
three or four I2C lines, depending on how the protocol is
implemented: one of these lines is a serial clock (SCL); another of the
lines is a serial data (SDA) line; another line is a ground (GND) line;
and another line is the voltage supply (VDD) line.

[0017] Because the bond wires corresponding to the I2C lines often
have different voltage potentials between them, they create electrical
fields. In addition, there is often some moisture in the air that
surrounds these bond wires because the modules often are not hermetically
sealed due to their compactness. Furthermore, free ions are often
available to migrate around the locations where the ends of the bond
wires attach to the respective bond pads on the substrate. The
combination of all of these effects can create impedance degradation and
electrical shorts between some of these adjacent bond wires and bond pads
that results in a failure of the module to operate properly, particularly
with respect to I2C communications, which are consistently in biased
conditions.

[0018] In accordance with the invention, a protective coating is applied
to the bond pads on the substrate and extends in between adjacent bond
pads. The protective coating has characteristics that (1) increase the
dielectric resistances between adjacent bond pads on the substrate, (2)
isolate the bond pads on the substrate from moisture in the environment,
and (3) prevents, or at least reduces, ion migration between adjacent
bond pads on the substrate. In this way, the protective coating prevents,
or at least reduces, corrosion growth that can lead to electrical shorts
between adjacent bond pads on the substrate. Illustrative embodiments
will now be described with reference to an example of a parallel optical
transmitter module to demonstrate the manner in which the protective
coating is applied in cases where the pitch between bond wires and bond
pads is very small. It should be noted, however, that the invention is
not limited to any particular type or configuration of an optical
communications module.

[0019] FIGS. 1-4 illustrate top perspective, top plan, front plan, and
side plan views, respectively, of a parallel optical communications
module 1 in accordance with an illustrative embodiment. In accordance
with this embodiment, the parallel optical communications module 1 is a
parallel optical transmitter. However, it should be noted that the
parallel optical communications module may be a parallel optical
transmitter, a parallel optical receiver or a parallel optical
transceiver. In the interest of brevity, illustrative embodiments of the
invention will be described with reference to the parallel optical
transmitter module 1. Those skilled in the art will understand the manner
in which the principles and concepts described herein in relation to the
parallel optical transmitter module 1 can be applied to parallel optical
receivers and parallel optical transceivers.

[0020] The parallel optical transmitter module 1 includes a mounting core
10 that serves as a mounting system for mounting at least the core
components of the transmitter module 1. In accordance with this
embodiment, the core components of the parallel optical transmitter
module 1 include a first laser diode driver IC 2, a second laser diode
driver IC 3, and a vertical cavity surface emitting laser (VCSEL) IC 4.
The laser diode driver ICs 2 and 3 and the VCSEL IC 4 are mounted on an
upper surface 10a of the mounting core 10. The laser diode driver ICs 2
and 3 are electrically connected to the VCSEL IC 4 by electrical
conductors 5, which are typically bond wires, to enable electrical
control signals and other electrical signals to be sent from the laser
diode driver ICs 2 and 3 to the VCSEL IC 4. The VCSEL IC 4 has a
plurality of VCSEL laser diodes (not shown) that produce a plurality of
respective optical data signals based on the electrical control signals
and respective electrical data signals provided to the VCSEL IC 4 by the
laser diode driver ICs 2 and 3. The electrical control signals control
the bias and modulation currents of the VCSEL laser diodes.

[0021] In the illustrative embodiment shown in FIG. 1, the laser diode
driver ICs 2 and 3 and the VCSEL IC 4 are arranged in a balanced laser
driver layout on the upper surface 10a of the mounting core 10. In the
balanced laser driver layout, half of the laser diodes of the VCSEL IC 4
are driven by laser diode driver IC 2 and the other half of the laser
diodes of the VCSEL IC 4 are driven by laser diode driver IC 3. It should
be noted, however, that it is not necessary to use the balanced laser
driver layout shown in FIG. 1.

[0022] The ICs and any other components that are mounted on the upper
surface 10a of the mounting core 10 may be arranged in any desired
layout. For example, the laser diode driver IC 2 could be used to drive
all of the laser diodes of the VCSEL IC 4, in which case the laser diode
driver IC 3 could be eliminated. Also, the invention is not limited with
respect to the types of laser diodes that are used. Laser diodes other
than VCELs (e.g., edge-emitting laser diodes) may be used for this
purpose. The invention also is not limited with respect to the types or
quantity of components that are mounted on the mounting core 10.

[0023] In the illustrative embodiment shown in FIG. 1, monitor photodiodes
7 are integrated into the laser diode driver ICs 2 and 3. These monitor
photodiodes 7 monitor the optical output levels of respective ones of the
laser diodes and produce corresponding electrical signals that are fed
back to control logic (not shown), which uses the feedback to adjust the
electrical control signals that are delivered by the laser diode driver
ICs 2 and 3 to the VCSEL IC 4. These control signals cause the bias
and/or modulation currents of the laser diodes to be adjusted such that
the average optical output power levels of the laser diodes are
maintained at substantially constant predetermined levels. However, the
monitor photodiodes 7 are optional and are not required by the parallel
optical communications device of the invention.

[0024] The mounting core 10 has a lower surface 10b that is attached to an
upper surface 20a of a substrate 20 of the parallel optical transmitter
1. The substrate 20 is a circuit board of some type, such as a land grid
array (LGA), for example. The substrate 20 has electrically-conductive
bond pads 21 (FIG. 3) on the upper surface 20a thereof. The bond pads 21
(FIG. 3) disposed on the upper surface 20a of the substrate 20 are
encapsulated in a protective coating 30, and therefore are not visible in
FIGS. 1, 2 and 4. The manner in which the protective coating 30
encapsulates the bond pads 21 (FIG. 3) will be described below in more
detail with reference to FIG. 5.

[0025] The bond pads 21 (FIG. 3) on the upper surface 20a of the substrate
20 are electrically coupled via electrically-conductive bond wires 36 to
respective electrically-conductive bond pads 37 of the laser diode driver
ICs 2 and 3. The ends of the bond wires 36 that are attached to the bond
pads 21 (FIG. 3) disposed on the upper surface 20a of the substrate 20
are typically also encapsulated in the protective coating 30, as will be
described below in more detail with reference to FIG. 5.

[0026] FIG. 5 illustrates a cross-sectional view of a portion of the
parallel optical transmitter module 1 shown in FIGS. 1-4 that
demonstrates the manner in which the protective coating 30 encapsulates
the bond pads 21 disposed on the upper surface 20a of the substrate 20
and the ends 36a of the bond wires 31 that are attached to the bond pads
21. As indicated above, the bond pads 21 may have a pitch that is very
small. This pitch, which corresponds to the distance between adjacent
bond pads 21, may be, for example, between about 15 and 40 micrometers
(microns). The protective coating 30 fills this space between the bond
pads 21 and encapsulates the bond pads 21 and the ends 36a of the bond
wires 36.

[0027] The invention is not limited to using any particular material for
the protective coating 30 except that it should have properties that
enable the coating 30 to: (1) adhere to the bond pads 21; (2) repel
moisture to an extent that moisture from the environment is not capable
of contacting the bond pads 21; (3) sufficiently increase the electrical
resistance between adjacent bond pads 21 to prevent electrical shorting
of adjacent bond pads 21; and (4) flow in between adjacent bond pads 21.
A variety of materials, such as epoxies, for example, are suitable for
this purpose. Many epoxies have dielectric constants that are
sufficiently high to prevent electrical shorting between adjacent bond
pads 21. Because the material must be able to flow in between adjacent
bond pads, if an epoxy with filler particles 31 (FIG. 5) is used for this
purpose, the filler particles 31 cannot have a size that is larger than
the pitch of the bond pads 21. Also, the material that is used for the
protective coating 30 must have a viscosity that is low enough to allow
the material to flow between the bond pads 21, but not so low that the
material flows onto other parts of the module 1 that should be free of
the material. The material also should not disturb the already-bonded
wires 36 or detrimentally affect the performance of the module 1. Persons
skilled in the art will understand how a material that has these
properties can be selected for use as the protective coating.

[0028] FIG. 6 illustrates a flowchart demonstrating the method for
applying the protective coating 30 in accordance with an illustrative
embodiment. The protective coating 30 may be applied manually or by an
automated process. In either case, application of the protective coating
30 should be performed in such a way that the integrity of the bond wires
36 is not detrimentally affected. Prior to applying the protective
coating 30, a die attachment process is performed to attach the ICs 2-4
to the mounting core 10, as indicated by block 50. It should be noted
that the mounting core 10 is optional and is shown here merely for
illustrative purposes. The ICs 2-4 may instead be mounted on a leadframe
or on heat sink devices that are attached to the substrate 20. After the
die attachment process has been performed, a wire bonding process is
performed to attach the bond wires 5 and 36 to the bond pads on the ICs
2-4 and on the substrate 20, as indicated by block 60. After the wire
bonding process has been performed, an encapsulation process is performed
to encapsulate the bond pads 21 disposed on the upper surface 20a of the
substrate 20 in the protective coating 30. It should be noted that
processes other than those represented by blocks 50, 60 and 70 may be
performed before, during and/or after the processes represented by blocks
50, 60 and 70 are performed.

[0029] As indicated above, the encapsulation process represented by block
70 may be performed manually by a person or automatically by a machine.
If the process is performed in automated fashion by a machine, a machine
vision system (not shown) will typically be used to align the module with
a dispensing tool that dispenses the protective coating 30. The
dispensing tool may be a tool that is specifically designed for this
purpose or it may be a tool that currently exists in the market. In the
latter case, for example, an inkjet shooter may be used to shoot the
coating material onto and in between the bond pads 21. The invention is
not limited to using any particular tools or systems to perform the
encapsulation process.

[0030] It should be noted that the invention has been described with
respect to illustrative embodiments for the purpose of describing the
principles and concepts of the invention. The invention is not limited to
these embodiments. For example, while the invention has been described
with reference to a particular module layout, the invention is not
limited to this particular layout. Also, while the invention has been
described with reference to a parallel optical transmitter in which all
channels are transmit channels, the parallel optical communications
module may instead include transmit and receive channels or only receive
channels. As will be understood by those skilled in the art in view of
the description being provided herein, many modifications may be made to
the embodiments described herein, and all such modifications are within
the scope of the invention.